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Consider the GBN protocol with a sender window size of 4 and a sequence number range of 1,024. Suppose that at time t, the next in-order packet that the receiver is expecting has a sequence number of k. Assume that the medium does not reorder messages. Answer the following questions: a. What are the possible sets of sequence numbers inside the sender’s window at time t? Justify your answer. b. What are all possible values of the ACK field in all possible messages currently propagating back to the sender at time t? Justify your answer.

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(a)

Consider the following data:

Window size (N) = 4

Range of sequence number = 1024

Case 1:

• Assume that the sequence number of the packet being expected by the receiver is k and assume that the receiver has received and acknowledged all the k-1 packets.

• The sender’s window will be in the range of [ k, k+N-1] sequence numbers when all the k-1 acknowledgements have been received without loss.

Case 2:

• If the sender does not receive any of the acknowledgements, then the sender’s window will be in the range of [ k-N, k-1] sequence numbers.

• Since it did not receive any acknowledgements, it will try to send all the k-1 and N packets.

• So, the sender’s window will be in the range of [ k-N, k-1] sequence numbers.

Hence, the possible sets of sequence numbers inside the sender’s window at time t are in the range [ k-N, k].

(b)

• When the receiver is waiting for a packet with sequence number k and if it starts receiving the N-1 earlier packets, then the possible values of the acknowledgement(ACK) field will be [ k-N, k-1].

• It is so because when the sender has not received the N ACKs, then the ACK messages may be propagating back.

• As the sender has sent all the k-N packets and received ACK for k-N-1 from the receiver, it will never send an ACK less than the n-N-1 ACK.

Therefore, all possible values of the ACK field in all messages currently propagating back to the sender at time t are in the range of ACK values and they will be in range between k-N-1 and k-1.


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